Method of producing a substrate having semiconductor-on-insulator structure with gettering sites

ABSTRACT

A substrate has a semiconductor-on-insulator structure. The substrate has a base substrate, an insulator layer provided on the base substrate, an active substrate provided on the insulator layer and having gettering sites, and an active layer provided on the active substrate and made of a semiconductor. The gettering sites under the active layer eliminate crystal defects and impurities generated in the active layer during the semiconductor device production in which elements are formed in the active layer.

This is a divisional of application Ser. No. 382,937 filed Jul. 21,1989, now U.S. Pat. No. 5,063,113.

BACKGROUND OF THE INVENTION

The present invention generally relates to substrates havingsemiconductor-on-insulator structures and production methods thereof,and more particularly to a substrate having a semiconductor-on-insulatorstructure with gettering sites and a production method thereof.

A substrate having a silicon-on-insulator (SOI) structure (hereinaftersimply referred to as an SOI substrate) is effectively used as asemiconductor crystalline substrate for realizing a high-speed operationof a semiconductor device.

Generally, when producing a semiconductor device having a semiconductorcrystalline substrate, it is necessary to carry out a gettering toeliminate crystal defects and impurities which are generated during aproduction process of the semiconductor device from an active layer inwhich semiconductor elements are formed. However, in the case of asemiconductor device having the SOI substrate, it is difficult torealize the gettering function due to its structure. This difficulty inrealizing the gettering function in the SOI substrate is an obstacle toobtaining a high performance in the semiconductor device having the SOIsubstrate.

As methods of forming the SOI substrate, there basically are twomethods. According to a first method, an SOI substrate is first formed,and a gettering site is formed thereafter. In FIG. 1A, the SOI substratecomprises a substrate 1, a silicon dioxide (SiO₂) layer 2 which isformed on the substrate 1, and a silicon (Si) layer which is formed onthe SiO₂ layer 2. This SOI substrate is subjected to a thermal processso as to form a gettering site 4 in the Si layer 3 as shown in FIG. 1B.However, the coefficients of thermal expansion of SiO₂ and Si aredifferent, and it is undesirable to subject the SOI substrate to thethermal process which is carried out at a relatively high temperature inthe order of 1400° C. for a relatively long time. In addition, becausethe thickness of the Si layer 3 is in the order of 5 microns and small,the gettering site 4 is small and unsatisfactory for carrying out theintended elimination of the crystal defects and impurities from adenuded zone 3a of the layer 3.

According to a second method, two stacked structures respectively madeup of a single crystal Si substrate and an SiO₂ layer are prepared andthe SiO₂ layers of the two stacked structures are bonded to form aso-called bonded substrate. This method is a more promising method thanthe first method due to recent improvements in bonding techniques. Butit is impossible to realize the gettering function in the SOI substratewhich is produced by this second method.

As methods of forming gettering sites in the substrate, there are theextrinsic gettering and the intrinsic gettering. The extrinsic getteringforms the gettering sites by applying an external distortion to a backsurface of the Si semiconductor substrate. On the other hand, theintrinsic gettering uses the Si semiconductor substrate which is formedby the Czochralski method, for example. Normally, the Si semiconductorsubstrate formed by the Czochralski method includes oxygen (O₂) in theorder of 1×10¹⁸ cm⁻³, and thus, the extrinsic gettering forms thegettering sites by subjecting the Si semiconductor substrate to athermal process to precipitate SiO₂.

In the case of the SOI substrate formed by the second method, the SiO₂layer exists at the central portion and it is impossible to employ theextrinsic gettering to form the gettering sites.

In addition, it is also impossible to employ the intrinsic gettering toform the gettering site in the SOI substrate which is formed by thesecond method for the following reasons. That is, in the SOI substrate,the thickness of a single crystal semiconductor layer in whichsemiconductor elements are formed needs to be in the order of 5 microns,and it is virtually impossible to form both an active layer for formingthe semiconductor elements and the gettering sites in this extremelythin single crystal semiconductor layer.

Normally, when realizing the gettering function in the Si semiconductorsubstrate which is formed by the Czochralski method, the Sisemiconductor substrate is subjected to a thermal process at atemperature of over 1000° C. so as to diffuse the O₂ in vicinities ofthe front and back surfaces of the Si semiconductor substrate and form adenuded zone. Then, a thermal process is carried out at a temperature ina range of 400° C. to 600° C. to form gettering sites. Next, a thermalprocess is carried out at a temperature of over 1000° C. to grow thegettering sites.

The semiconductor elements are formed in the denuded zone, but thethickness of the denuded zone is in the order of 20 microns. Hence, evenwhen a stacked structure 10 made up of a base substrate 11 and a SiO₂layer 12 is bonded to another stacked structure 15 made up of a Siactive substrate 16 and SiO₂ layers 17a and 17b and having getteringsites 18 formed by the intrinsic gettering as shown in FIG. 2A, the SiO₂layer 17a and the Si substrate 16 must be polished so as to leave anactive layer which has a thickness in the order of 5 microns. As shownin FIG. 2B, the gettering site 18 will be removed by the polishing asindicated by a phantom line and only the active layer remains accordingto this method. Therefore, in the case of the SOI substrate formed bythe second method, it is also impossible to employ the intrinsicgettering to form the gettering site.

Therefore, in the case of the SOI substrate formed by the second method,it is impossible to form a gettering site.

Because it is conventionally impossible to realize the getteringfunction in the SOI substrate, there are problem in that it isimpossible to eliminate by the gettering the crystal defects andimpurities generated during the production process of the semiconductordevice which has the SOI substrate, and there is no means to preventdeterioration in the characteristics of the semiconductor elements whichare formed in the active layer.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful substrate having a semiconductor-on-insulatorstructure and a production method thereof in which the problemsdescribed above are eliminated.

Another and more specific object of the present invention is to providea substrate having a semiconductor-on-insulator structure comprising abase substrate, an insulator layer provided on the base substrate, anactive substrate provided on the insulator layer and having getteringsites, and an active layer provided on the active substrate and made ofa semiconductor. According to the substrate of the present invention, itis possible to satisfactorily eliminate crystal defects and impuritieswhich are generated in the active layer during the production process ofa semiconductor device by the provision of the gettering sites under theactive layer. Hence, it is possible to realize in the active layer asemiconductor element which has a satisfactory characteristic and isfree of defects.

Still another object of the present invention is to provide a method ofproducing a substrate having a semiconductor-on-insulator structurecomprising the steps of preparing a stacked structure having a basesubstrate and an insulator layer formed thereon, forming an activesubstrate on the insulator layer of the stacked structure which activesubstrate has gettering sites, and forming an active layer made of asemiconductor on the active substrate. According to the method of thepresent invention, it is possible to easily form the gettering sites inthe substrate which has the semiconductor-on-insulator structure whichincludes the SOI structure. Hence, it is possible to improve thecharacteristics of semiconductor elements which are formed in the activelayer.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B and FIGS. 2A and 2B respectively are cross sectionalviews for explaining problems of forming gettering sites in an SOIsubstrate;

FIGS. 3A through 3E respectively are cross sectional views forexplaining a first embodiment of a production method according to thepresent invention for producing a first embodiment of a substrate havinga semiconductor-on-insulator structure according to the presentinvention;

FIGS. 4A through 4G respectively are cross sectional views forexplaining a second embodiment of the production method according to thepresent invention for producing a second embodiment of the substratehaving the semiconductor-on-insulator structure according to the presentinvention;

FIGS. 5A through 5D respectively are cross sectional views forexplaining a third embodiment of the production method according to thepresent invention for producing a third embodiment of the substratehaving the semiconductor-on-insulator structure according to the presentinvention; and

FIG. 6 is a cross sectional view showing a CMOS field effectsemiconductor device which is applied with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be given of a first embodiment of a production methodaccording to the present invention for producing a first embodiment of asubstrate having a semiconductor-on-insulator structure according to thepresent invention, by referring to FIGS. 3A through 3E.

A Si semiconductor active substrate 21 shown in FIG. 3A is prepared bythe Czochralski method, for example. Of course, this active substrate 21includes O₂.

The active substrate 21 is subjected to a thermal process to formgettering sites 22 which are made of SiO₂ as shown in the FIG. 3B. Nodenuded zone is formed by this thermal process.

The active substrate 21 having the gettering sites 22 is subjected to athermal process in an oxidizing atmosphere to form a SiO₂ layer 23 on asurface of the active substrate 21 as shown in FIG. 3C.

A Si semiconductor base substrate 24 is subjected to a thermal processin an oxidizing atmosphere to form a SiO₂ layer 25 on a surface of thebase substrate 24, and the SiO₂ layer 25 is bonded to the SiO₂ layer 23as shown in FIG. 3D. The active substrate 21 is polished to apredetermined thickness. As a result, a layer of the gettering sites 22is formed in the active substrate 21 on top of the SiO₂ layer 23.

Finally, a Si semiconductor active layer 26 is formed on the activesubstrate 21 having the layer of the gettering sites 22 as shown in FIG.3E. For example, the active layer 26 is formed by an epitaxial growthprocess.

Accordingly, this embodiment of the substrate having asemiconductor-on-insulator structure according to the present inventioncomprises a base stacked structure which is made up of the basesubstrate 24 and the SiO₂ layer 25, and an active stacked structurewhich is made up of the active substrate 21 and the SiO₂ layer 23. Thebase stacked structure and the active stacked structure are bonded bybonding the SiO₂ layers 25 and 23, and the active layer 26 is formed onthe active substrate 21 which has the layer of the gettering sites 22.

Because the layer of the gettering sites 22 is provided under the activelayer 26, the crystal defects and impurities generated within the activelayer 26 during the production process of the semiconductor device canbe eliminated by the gettering function of the layer of the getteringsites 22. For this reason, it is possible to form semiconductor elementshaving satisfactory characteristics in the active layer 26.

Next, a description will be given of a second embodiment of theproduction method according to the present invention for producing asecond embodiment of the substrate having a semiconductor-on-insulatorstructure according to the present invention, by referring to FIGS. 4Athrough 4G.

A base substrate 31 shown in FIG. 4A is prepared. In this embodiment,the substrate 31 is made of a single crystal Si semiconductor. However,the base substrate 31 may be made of other materials such aspolysilicon, compound semiconductor, ceramic, refractory metal, andquartz.

The base substrate 31 is subjected to a thermal process in an oxidizingatmosphere to form a SiO₂ layer 32 as shown in FIG. 4B. The SiO₂ layer32 is formed to a thickness in a range of 1000 Å to 1 micron. When thebase substrate 31 is made of a material other than the single crystal Sisemiconductor, a SiO₂ layer can be formed directly on the base substrateby an oxidation process carried out after growing a polysilicon layer, achemical vapor deposition or the like.

An active substrate 33 shown in FIG. 4C is prepared by the Czochralskimethod. The active substrate 33 includes 1×10¹⁷ or more of O₂.

For example, the active substrate 33 is subjected to a thermal processat 450° C. for 5 hours and then to a thermal process at 1100° C. for 1hour to precipitate and grow gettering sites 35 made of SiO₂ as shown inFIG. 4D. Then, a thermal process is carried out in an oxidizingatmosphere to form a SiO₂ layer 34 on both sides of the substrate 33 toa thickness in the order of 2000 Å.

The substrates 31 and 33 are bonded by bonding SiO₂ layers 32 and 34 asshown in FIG. 4E. For example, this bonding is achieved under thefollowing bonding conditions.

Temperature: 1000° C.

Pulse voltage: ±300 V

Time: 5 minutes

The active stacked structure made up of the SiO₂ layer 34 and the activesubstrate 33 are polished to a thickness of 3 microns, for example. As aresult, only the layer of the gettering sites 35 of the active substrate33 remains on the SiO₂ layer 34 which is bonded to the SiO₂ layer 32 asshown in FIG. 4F. For example, a polishing agent comprising Al₂ O₃, achemical etchant or the like may be used for the polishing process.

A Si semiconductor active layer 36 is formed on the active substrate 33which is constituted by the layer of the gettering sites 35 as shown inFIG. 4G. For example, the active layer 36 is formed by an epitaxialgrowth process to a thickness of 1 micron. As described before, thecrystal defects and impurities generated in the active layer 36 duringthe production process of the semiconductor device are effectivelyeliminated by the provision of the layer of the gettering sites 35 underthe active layer 36.

The effects of the present invention were confirmed by forming a metaloxide semiconductor (MOS) diode in the substrate of the presentinvention and comparing characteristics thereof with a MOS diode formedin the conventional SOI substrate. An SOI substrate having a layer ofgettering sites with a thickness of 1 micron and an active layer with athickness of 2 microns was used as the substrate of the presentinvention, while an SOI substrate having no layer of gettering sites andan active layer with a thickness of 3 microns was used as theconventional SOI substrate. Identical MOS diodes were formed in therespective active layers of the two SOI substrates, and life times ofminority carriers were measured. The life time of minority carriers inthe substrate of the present invention was 800±100 microseconds, whilethe life time of minority carriers in the conventional SOI substrate was500±200 microseconds. It was thus confirmed that the absolute value ofthe life time of minority carriers in the present invention isconsiderably longer compared to that obtained in the conventional SOIsubstrate, and the deviation in the life time of minority carriers inthe present invention is small compared to that of the conventional SOIsubstrate.

Accordingly, it is clear from the above measurements that a junctionleak current can be suppressed to an extremely small value by use of theSOI substrate of the present invention. In addition, in the case of adynamic random access memory (DRAM) employing the substrate of thepresent invention, it is evident that the information storagecharacteristic can be improved. In other words, the characteristic ofthe elements formed in the SOI substrate and the production yield canboth be improved.

In the embodiment described heretofore, the substrate has the SOIstructure. However, the present invention is of course applicable to anysubstrate having the semiconductor-on-insulator structure. For example,it is possible to use gallium arsenide (GaAs) for the active substratein place of Si and form a layer of gettering sites in the GaAs activesubstrate. In this case, the gettering sites may be formed in the GaAsactive substrate by an ion implantation process or the like after theGaAs active substrate is formed on the insulator layer. Furthermore, thebase substrate may be made of a material other than Si such aspolysilicon, silicon carbide, metal, and insulator. The insulator layeris also not limited to the SiO₂ layer, and the insulator layer may bemade of any insulator such as glass and phosphosilicate glass (PSG).

FIGS. 5A through 5C show production steps of a third embodiment of theproduction method according to the present invention for producing athird embodiment of the substrate having the semiconductor-on-insulatorstructure according to the present invention. First, a GaAs activesubstrate (layer) 102 is formed on an insulator layer 101 which isprovided on top of a base substrate 100 as shown in FIG. 5A. The stackedstructure shown in FIG. 5A is subjected to an ion implantation as shownin FIG. 5B to form gettering sites 103 in the GaAs active substrate 102as shown in FIG. 5C. A GaAs active layer 104 is formed on the GaAsactive substrate 102 which has the gettering sites 103, as shown in FIG.5D.

As a modification of this embodiment, it is possible to prepare a GaAsactive substrate 102 which already has the gettering sites 103 formedtherein and bond this GaAs active substrate on the insulator layer 101.

Next, a description will be given of a complementary metal oxidesemiconductor (CMOS) field effect semiconductor device which is appliedwith the present invention, by referring to FIG. 6. In FIG. 6, thesubstrate employs the SOI structure as the semiconductor-on-insulatorstructure. In FIG. 6, those parts which are substantially the same asthose corresponding parts in FIGS. 4A through 4G are designated by thesame reference numerals, and a description thereof will be omitted.

In FIG. 6, a p-type well 36P and an n-type well 36N are formed in theactive layer 36. A SiO₂ element isolation layer 37 is formed to the SiO₂layer 34 to isolate elements formed in the active layer 36. An n-channeltransistor formed in the active layer 36 comprises a SiO₂ gate insulatorlayer 38N, a polysilicon gate electrode 39N, an n⁺ -type source region40N, and an n⁺ -type drain region 41N. A p-channel transistor formed inthe active layer 36 comprises a SiO₂ gate insulator layer 38P, apolysilicon gate electrode 39P, a p⁺ -type source region 40P, and a p⁺-type drain region 41P.

As may be seen from FIG. 6, this CMOS field effect semiconductor devicehas the layer of the gettering sites 35 under the p-type well 36P andthe n-type well 36N. Hence, it is possible to eliminate the crystaldefects and impurities generated during the production process. Forexample, it is thus possible to prevent a junction leak and adeterioration of the withstand voltage of the gate insulator layers 38Nand 38P. Of course, the n-channel transistor and the p-channeltransistor are completely isolated by the SiO₂ as in the case of theconventional SOI structure. Therefore, it is possible to easily realizea latch up free device. In addition, since the source regions 40N and40P and the drain regions 41N and 41P are in contact with the elementisolation layer 37 which is made of SiO₂, it is possible to suppress thespreading of the depletion layer, make the stray capacitance small andthus realize a high switching speed.

In the described embodiments, the bonded substrate is used as thesubstrate having the semiconductor-on-insulator structure. However, itis not essential that the layer of gettering sites is bonded on theinsulator layer of the substrate. It is possible to form the getteringsites in a layer after the layer is formed or bonded on the insulatorlayer.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A method of producing a substrate having asemiconductor-on-insulator structure comprising the steps of:preparing astacked structure having a base substrate and an insulator layer formedthereon; forming an active substrate on the insulator layer of saidstacked structure, said active substrate having gettering sites; andforming an active layer on said active substrate, said active layerbeing made of a semiconductor.
 2. The method as claimed in claim 1wherein said base substrate is made of a material selected from a groupincluding silicon, polysilicon, silicon carbide, metal, and insulator.3. The method as claimed in claim 1 wherein said insulator layer is madeof a material selected from a group including silicon dioxide, glass andphosphosilicate glass.
 4. The method as claimed in claim 1 wherein saidactive substrate is made of a material selected from a group ofsemiconductors including silicon and gallium arsenide.
 5. The method asclaimed in claim 1 wherein said active layer is made of a materialselected from a group including silicon and gallium arsenide.
 6. Themethod as claimed in claim 1 wherein said insulator layer is made ofsilicon dioxide, said active substrate is made of silicon, and saidactive layer is made of silicon.
 7. The method as claimed in claim 1which further comprises the step of forming metal oxide semiconductorelements in said active layer.
 8. The method as claimed in claim 1wherein said step of forming said active substrate on said insulatorlayer includes a substep of forming an insulator layer on said activesubstrate and a substep of bonding the insulator layer of said activesubstrate on the insulator layer of said stacked structure.
 9. Themethod as claimed in claim 1 wherein said step of forming said activesubstrate on said insulator layer includes a substep of preparing saidactive substrate by Czochralski method, a substep of forming thegettering sites in said active substrate by a thermal process, a substepof forming an insulator layer on said active substrate by a thermalprocess, and a substep of bonding the insulator layer of said activesubstrate on the insulator layer of said stacked structure.
 10. Themethod as claimed in claim 1 wherein said step of forming said activelayer grows said active layer by an epitaxial growth process.
 11. Themethod as claimed in claim 1 wherein said step of forming said activesubstrate forms the gettering sites in said active substrate beforeforming said active substrate on said insulator layer.
 12. The method asclaimed in claim 1 wherein said step of forming said active substrateforms the gettering sites in said active substrate after forming saidactive substrate on said insulator layer.